1. Field of the Invention
The present invention relates to a dialytic apparatus for single-needle dialysis and in particular to a dialytic apparatus having an expansion chamber interposed between a first pump and a second pump in the artery conduit leading to the blood-inlet port of the dialyzer.
2. Description of Related Art
When a single-needle hemodialysis is performed, the patient is punctured by a single needle at a single spot. During a first phase, blood is taken away from the patient through this needle and conveyed to the dialyzer. During a second phase the blood is returned from the dialyzer to the patient.
A dialytic apparatus is known (German Patent Specification No. 22 36 433) which comprises a closed system in which the single one-bore needle is alternately connected to the artery branch and the vein branch while the respective other branch is shut off. During the first phase, the vein branch is closed. The pump conveys blood from the patient via the artery branch to the dialyzer. The first phase is finished when the pressure at the blood outlet port of the dialyzer has reached an upper limiting value. In the succeeding second phase, the artery conduit is shut off, and the vein conduit is opened. Subsequently, under the release of pressure, the blood flows back into the patient via the vein conduit and the needle. This known system is based on time-dependent variations of the pressures at the dialyzer, which variations are used for controlling the device. However, variations of pressure at the dialyzer membrane greatly disturb the function of the dialyzer itself, since the variations influence the ultrafiltration rate and prevent a balanced ultrafiltration through the dialyzer membrane. Additionally, if there is a standstill of the blood flow at the dialyzer membrane, the continuing ultrafiltration can cause a decrease of the vein return pressure below the permittd limiting value, so that a corresponding alarm device is actuated.
In a further known dialytic apparatus (U.S. Pat. No. 4,614,590), an expansion chamber is interposed in a parallel branch of a single pump. During the first phase, the pump conveys blood into the expansion chamber while the blood passage through the dialyzer is blocked. During the second phase, the pump conveys the blood out of the expansion chamber to the dialyzer, the artery conduit being shut off. Here again, there are considerably changes of the transmembrane pressure at the dialyzer membrane and an intermittently interrupted discontinuous dialysis. A lower limiting value of the vein return pressure cannot be easily maintained.
It is therefore an object of the present invention to provide a single needle dialytic apparatus in which the pressure exerted on the dialyzer membrane from the blood side is not subjected to large changes when the vein conduit is shut off.